Fibers of uhmwpe and a process for producing thereof

ABSTRACT

The invention relates to a process for producing gel-spun ultra high molecular weight polyethylene (UHMWPE) fibres having high tensile strengths and improved creep rates wherein the UHMWPE used in said process is characterized by a difference in the phase angle according to Formula 1 
       Δδ=δ 0.001 −δ 100   (1)
 
     of at most 42°,
 
wherein δ 0.001  is the phase angle at an angular frequency of 0.001 rad/sec; and
         δ 100  is the phase angle at an angular frequency of 100 rad/sec as measured with a frequency sweep dynamic rheological technique at 150° C. on a 10% solution of UHMWPE in paraffin oil, provided that δ 100  is at most 18°. The invention further relates to gel-spun UHMWPE fibres produced thereof. The gel-spun UHMWPE fibres of the invention have a tensile strength of at least 4 GPa, and a creep rate of at most 5×10 −7  sec −1  as measured at 70° C. under a load of 600 MPa. The gel-spun UHMWPE fibres produced thereof are useful in a variety of applications, the invention relating in particular to ropes, medical devices, composite articles and ballistic-resistant articles containing said UHMWPE fibres.

This application is a divisional of commonly owned copending U.S.application Ser. No. 12/681,700, filed Jul. 16, 2010 (now U.S. Pat. No.______), which is the national phase application under 35 USC §371 ofPCT/EP2008/008418, filed Oct. 6, 2008, and claims the benefit ofpriority of EP 07019507.8, filed Oct. 5, 2007, the entire contents ofwhich are hereby incorporated by reference.

The invention relates to a process for producing gel-spun ultra highmolecular weight polyethylene (UHMWPE) fibres having high tensilestrengths and improved creep rates and to gel-spun UHMWPE fibresproduced thereof. The gel-spun UHMWPE fibres produced thereof are usefulin a variety of applications, the invention relates in particular toropes, medical devices, composite articles and ballistic-resistantarticles containing said UHMWPE fibres.

A process for producing gel-spun UHMWPE fibres having high tenacitiesand improved creep resistance is known for example from EP1,699,954 theprocess comprising the steps of:

-   -   a) Preparing an UHMWPE solution in a solvent, the UHMWPE having        an intrinsic viscosity in decalin at 135° C. of at least 5 dl/g;    -   b) Spinning the solution of step a) through a spinneret        containing multiple spinholes into an air gap to form fluid        filaments;    -   c) Cooling the fluid filaments to form solvent-containing gel        filaments; and    -   d) Removing at least partly the solvent from the gel filaments        to form solid filaments before and/or during drawing the solid        filaments.

The obtained UHMWPE fibres had creep rates as low as 1×10⁻⁶ sec⁻¹ asmeasured at 70° C. under a load of 600 MPa and tensile strengths as highas 4.1 GPa.

It is also known that a lowering of the creep rates of UHMWPE fibres maybe achieved with a gel-spinning process wherein a highly branched UHMWPEis used, i.e. an UHMWPE having branches longer than a methyl branch, asfor example ethyl, propyl and the like, or having a high amount of saidbranches or a combination thereof. However, said highly branchedpolyethylenes impair the drawing properties of the spun UHMWPE fibresand therefore, fibres having inferior tensile properties are produced.

On the other hand, it is known that by using less branched UHMWPE, i.e.polyethylenes having a more linear configuration meaning a low amount ofbranches or short branches, e.g. methyl branches, fibres with improvedtensile properties can be produced. However, these fibres show poorcreep properties. Therefore, because the creep and tensile propertiesare not concurrent properties, it is not by any means trivial for anyoneskilled in the art to obtain UHMWPE fibers with a low creep rate as wellas high tensile strength.

The object of the invention is therefore to fulfil the need for UHMWPEfibres having a combination of high tensile strength and low creep rate,combination that is not met by any of the existent UHMWPE fibres and fora process for the preparation thereof.

The object of the invention was achieved with a process of producinggel-spun UHMWPE fibres characterized in that the UHMWPE is characterizedby a difference in the phase angle according to Formula 1

Δδ=δ_(0.001)−δ₁₀₀  (1)

of at most 42°,wherein δ_(0.001) is the phase angle at an angular frequency of 0.001rad/sec and

δ₁₀₀ is the phase angle at an angular frequency of 100 rad/sec asmeasured with a frequency sweep dynamic rheological technique at 150° C.on a 10% solution of UHMWPE in paraffin oil, provided that δ₁₀₀ is atmost 18°.

Surprisingly, the inventors found that by using an UHMWPE having aΔδ=δ_(0.001)−δ₁₀₀ difference as defined by claim 1 in the gel-spinningprocess of the invention, novel UHMWPE fibres were obtained having acombination of tensile strengths and creep rates which to inventors'knowledge was never achieved hitherto.

It was also surprisingly observed that it was possible to apply a higheroverall draw ratio (DR_(overall)) to the spun fibres in the process ofthe invention without the occurrence of breakages as compared to theDR_(overall) applied to fibres made of known UHMWPE or previouslyreported in the state of the art. By DR_(overall) is herein understoodthe multiplication of the draw ratios applied to the fibres at differentstages in the process of the invention, i.e. the draw ratios applied tofluid, gel and solid fibres. Accordingly,DR_(overall)=DR_(fluid)×DR_(gel)×DR_(solid).

Preferably, the DR_(overall) applied to the UHMWPE fibres of theinvention, is at least 5000, more preferably at least 8000, even morepreferably at least 12.000, yet even more preferably at least 15.000,yet even more preferably at least 20.000, yet even more preferably atleast 25.000, yet even more preferably at least 30.000, most preferablyat least 35.000.

The advantage of applying such high DR_(overall) in the process of theinvention, is that unique UHMWPE fibres were obtained having evenfurther improved creep rates and/or tensile strengths.

A further advantage of the process of the invention is that a higherdrawing rate can be used to draw the UHMWPE fibres of the invention,improving the production output and decreasing the production time,therefore, making the process of the invention more attractiveeconomically. By drawing rate is herein understood the drawing ratiodivided by the time in seconds needed to achieve said drawing ratio.

Preferably, the Δδ0 difference characterizing the UHMWPE used in theprocess of the invention is at most 40°, more preferably at most 38°,even more preferably at most 36°, most preferably at most 35° providedthat δ₁₀₀ is at most 18°. Preferably, said Δδ difference is at least 5°,more preferably at least 15°, most preferably at least 25°. Preferably,said Δδ difference has the above mentioned values provided that δ₁₀₀ isat most 16°, more preferably at most 14°, even more preferably at most12°, most preferably at most 10°. Preferably, δ₁₀₀ is at least 2°, morepreferably at least 4°, most preferably at least 6°.

The UHMWPE used in the process of the invention has an intrinsicviscosity (IV) as measured on solution in decalin at 135° C., of atleast 5 dl/g, preferably at least 10 dl/g, more preferably at least 15dl/g, most preferably at least 21 dl/g. Preferably, the IV is at most 40dl/g, more preferably at most 30 dl/g, even more preferably at most 25dl/g.

In a preferred embodiment, said UHMWPE comprises per thousand carbonatoms between 0.1 and 1.3, even more preferably between 0.2 and 1.2, yeteven more preferably between 0.25 and 0.9, yet even more preferablybetween 0.3 and 0.6, most preferably between 0.3 and 0.5 methyl groupscorresponding to ethyl side groups. More preferably, said UHMWPEcomprises per thousand carbon atoms between 0.08 and 0.7, even morepreferably between 0.08 and 0.5, yet even more preferably between 0.09and 0.4, yet even more preferably between 0.1 and 0.3, yet even morepreferably between 0.15 and 0.3, most preferably between 0.2 and 0.3 ofmethyl end groups. By methyl end groups is herein understood methylgroups corresponding to ends of the UHMWPE chains and to ends of longchain branches (LCB) of the UHMWPE chains. By LCB are herein understoodbranches longer than an ethyl group, e.g. propyl, butyl, hexyl andlonger branches.

Preferably, the total amount of methyl groups per thousand carbon atomsobtained by adding the amount per thousand carbon atoms of methyl groupscorresponding to ethyl side groups and the amount per thousand carbonatoms of methyl end groups is between 0.3 and 2, more preferably thetotal is between 0.4 and 1.7, even more preferably between 0.45 and 1.3,yet even more preferably between 0.5 and 0.9, most preferably between0.5 and 0.7.

It was surprisingly found that by using the UHMWPE of this preferredembodiment in the process of the invention, the combination of tensilestrength and creep rate and in particular the creep rate of the UHMWPEfibers of the invention is further improved.

The UHMWPE solution is preferably prepared with a concentration of atleast 3 mass-%, more preferably of at least 5 mass-%, even morepreferably at least 8 mass-%, most preferably at least 10 mass-%. TheUHMWPE solution, preferably has a concentration of at most 30 mass-%,more preferably at most 25 mass-%, even more preferably at most 20mass-%, most preferably at most 15 mass-%. To improve processability, alower concentration is preferred the higher the molar mass of thepolyethylene is. Preferably, the concentration is between 3 and 15mass-% for UHMWPE with IV in the range 15-25 dl/g.

To prepare the UHMWPE solution, any of the known solvents suitable forgel spinning the UHMWPE may be used. Suitable examples of solventsinclude aliphatic and alicyclic hydrocarbons, e.g. octane, nonane,decane and paraffins, including isomers thereof; petroleum fractions;mineral oil; kerosene; aromatic hydrocarbons, e.g. toluene, xylene, andnaphthalene, including hydrogenated derivatives thereof, e.g. decalinand tetralin; halogenated hydrocarbons, e.g. monochlorobenzene; andcycloalkanes or cycloalkenes, e.g. careen, fluorine, camphene, menthane,dipentene, naphthalene, acenaphthalene, methylcyclopentandien,tricyclodecane, 1,2,4,5-tetramethyl-1,4-cyclohexadiene, fluorenone,naphtindane, tetramethyl-p-benzodiquinone, ethylfuorene, fluorantheneand naphthenone. Also combinations of the above-enumerated solvents maybe used for gel spinning of UHMWPE, the combination of solvents beingalso referred to for simplicity as solvent. In a preferred embodiment,the solvent of choice is not volatile at room temperature, e.g. paraffinoil. It was also found that the process of the invention is especiallyadvantageous for relatively volatile solvents at room temperature, asfor example decalin, tetralin and kerosene grades. In the most preferredembodiment the solvent of choice is decalin.

According to the invention, the UHMWPE solution is formed into fluidfilaments by spinning said solution through a spinneret containingmultiple spinholes. As used herein, the term “fluid filament” refers toa fluid-like filament containing a solution of UHMWPE in the solventused to prepare said UHMWPE solution, said fluid filament being obtainedby extruding the UHMWPE solution through the spinneret, theconcentration of the UHMWPE in the extruded fluid filaments being thesame or about the same with the concentration of the UHMWPE solutionbefore extrusion. By spinneret containing multiple spinholes is hereinunderstood a spinneret containing preferably at least 10 spinholes, morepreferably at least 50, even more preferably at least 100, yet even morepreferably at least 300, most preferably at least 500. Preferably thespinneret contains at most 5000, more preferably 3000, most preferably1000 spinholes.

Preferably, the spinning temperature is between 150° C. and 250° C.,more preferably it is chosen below the boiling point of the spinningsolvent. If for example decaline is used as spinning solvent thespinning temperature is preferably at most 190° C., more preferably atmost 180° C., most preferably at most 170° C. and preferably at least115° C., more preferably at least 120° C., most preferably at least 125°C. In case of paraffin, the spinning temperature is preferably below220° C., more preferably between 130° C. and 195° C.

In a preferred embodiment, each spinhole of the spinneret has a geometrycomprising at least one contraction zone. By contraction zone is hereinunderstood a zone with a gradual decrease in diameter with a cone anglein the range 8-75° from a diameter D_(O) to D_(n) such that a draw ratioDR_(sp) is achieved in the spinhole. Preferably, the spinhole furthercomprises at least one zone of constant diameter with a length/diameterratio L_(n)/D_(n) of at most 50 downstream of the contraction zone. Morepreferably L_(n)/D_(n) is at most 40, even more preferably at most 25,most preferably at most 10 and preferably at least 1, more preferably atleast 3, most preferably at least 5. L_(n) is the length of the zonewith constant diameter D_(n). Preferably, the ratio D_(O)/D_(n) is atleast 2, more preferably at least 5, even more preferably at least 10,yet even more preferably at least 15, most preferably at least 20.Preferably, the cone angle is at least 10°, more preferably at least12°, even more preferably at least 15°. Preferably, the cone angle is atmost 60°, more preferably at most 50°, even more preferably at most 45°.

The diameter of the spinhole is herein meant to be the effectivediameter, i.e. for non-circular or irregularly shaped spinholes, thelargest distance between the outer boundaries of the spinhole.

With cone angle is herein meant the maximum angle between the tangentsto opposite wall surfaces in the contraction zone of the spinhole. Forexample, for a conical or tapered contraction zone, the cone anglebetween the tangents is constant, whereas for a so-called trumpet typeof contraction zone the cone angle between the tangents will decreasewith decreasing diameter. For a wineglass type of contraction zone theangle between the tangents passes through a maximum value.

The draw ratio in the spinholes DR_(sp) is represented by the ratio ofthe solution flow speed at the initial cross-section and at the finalcross-section of the contraction zone, which is equivalent to the ratioof the respective cross-sectional areas. In case of contraction zonehaving the shape of a frustum of a circular cone, DR_(sp) is equal tothe ratio between the square of the initial and final diameters, i.e.=(D_(O)/D_(n))².

Preferably, D_(O) and D_(n) are chosen to yield a DR_(sp) of at least 5,more preferably at least 10, even more preferably at least 15, yet evenmore preferably at least 20, yet even more preferably at least 25, mostpreferably at least 30.

The fluid filaments formed by spinning the UHMWPE solution through thespinneret are extruded into an air gap, and then into a cooling zonefrom where they are picked-up on a first driven roller. Preferably, thefluid filaments are stretched in the air gap with a drawing ratioDR_(ag) of at least 15 by choosing an angular speed of the first drivenroller such that said roller's surface velocity exceeds the flow rate ofthe UHMWPE solution issued form the spinneret. The draw ratio in the airgap, DR_(ag), is more preferably at least 20, even more preferably atleast 25, yet even more preferably at least 30, yet even more preferablyat least 35, yet even more preferably at least 40, yet even morepreferably at least 50, most preferably at least 60.

In a preferred embodiment, the DR_(sp) and DR_(ag) are chosen to yield atotal draw ratio of the fluid UHMWPE filaments,DR_(fluid)=DR_(sp)×DR_(ag) of at least 150, more preferably at least250, even more preferably at least 300, yet even more preferably atleast 350, yet even more preferably at least 400, yet even morepreferably at least 500, yet even more preferably at least 600, yet evenmore preferably at least 700, most preferably at least 800. It wassurprisingly found that it was possible to subject the fluid filamentscomprising the UHMWPE of the invention to a higher DR_(fluid) than itwas possible heretofore, while keeping the occurrence of breakages atthe same level.

Correspondingly, when the fluid UHMWPE filaments were subjected to aDR_(fluid) equally as large with those previously applied in the stateof the art, the breakages occurring to fluid filaments were reduced.

The length of the air gap is preferably at least 1 mm, more preferablyat least 3 mm, even more preferably at least 5 mm, yet even morepreferably at least 10 mm, yet even more preferably at least 15 mm, yeteven more preferably at least 25 mm, yet even more preferably at least35 mm, yet even more preferably at least 25 mm, yet even more preferablyat least 45 mm, most preferably at least 55 mm. The length of the airgap is preferably at most 200 mm, more preferably at most 175 mm, evenmore preferably at most 150 mm, yet even more preferably at most 125 mm,yet even more preferably at most 105 mm, yet even more preferably atmost 95 mm, most preferably at most 75 mm.

Cooling, also known as quenching, the fluid filaments after exiting theair-gap to form solvent-containing gel filaments, may be performed in agas flow and/or in a liquid cooling bath. Preferably, the cooling bathcontains a cooling liquid that is a non-solvent for UHMWPE and morepreferably a cooling liquid that is not miscible with the solvent usedfor preparing the UHMWPE solution. Preferably, the cooling liquid flowssubstantially perpendicular to the filaments at least at the locationwhere the fluid filaments enter the cooling bath, the advantage thereofbeing that the drawing conditions can be better defined and controlled.

By air-gap is meant the length traveled by the fluid filaments beforethe fluid filaments are converted into solvent-containing gel filamentsif gas cooling is applied, or the distance between the face of thespinneret and the surface of the cooling liquid in the liquid coolingbath. Although called air-gap, the atmosphere can be different than air;e.g. as a result of a flow of an inert gas like nitrogen or argon, or asa result of solvent evaporating from filaments or a combination thereof.

As used herein, the term “gel filament” refers to a filament which uponcooling develops a continuous UHMWPE network swollen with the spinningsolvent. An indication of the conversion of the fluid filament into thegel filament and the formation of the continuous UHMWPE network may bethe change in filament's transparency upon cooling from a translucentUHMWPE filament to a substantially opaque filament, i.e. the gelfilament.

Preferably, the temperature to which the fluid filaments are cooled isat most 100° C., more preferably at most 80° C., most preferably at most60° C. Preferably, the temperature to which the fluid filaments arecooled is at least 1° C., more preferably at least 5° C., even morepreferably at least 10° C., most preferably at least 15° C.

In a preferred embodiment the solvent-containing gel filaments are drawnin at least one drawing step with a draw ratio DR_(gel) of at least1.05, more preferably at least 1.5, even more preferably at least 3, yeteven more preferably at least 6, most preferably at least 10. Thedrawing temperature of the gel filaments is preferably between 10° C.and 140° C., more preferably between 30° C. and 130° C., even morepreferably between 50° C. and 130° C., yet even more preferably between80° C. and 130° C., most preferably between 100° C. and 120° C.

Subsequently to forming the gel filaments, said gel filaments aresubjected to a solvent removal step wherein the spinning solvent is atleast partly removed from the gel filaments to form solid filaments. Theamount of residual spinning solvent, hereafter residual solvent, left inthe solid filaments after the extraction step may vary within largelimits, preferably the residual solvent being in a mass percent of atmost 15% of the initial amount of solvent in the UHMWPE solution, morepreferably in a mass percent of at most 10%, most preferably in a masspercent of at most 5%.

The solvent removal process may be performed by known methods, forexample by evaporation when a relatively volatile spinning solvent, e.g.decaline, is used to prepare the UHMWPE solution or by using anextraction liquid, e.g. when paraffin is used, or by a combination ofboth methods. Suitable extraction liquids are liquids that do not causesignificant changes in the UHMWPE network structure of the UHMWPE gelfibres, for example ethanol, ether, acetone, cyclohexanone,2-methylpentanone, n-hexane, dichloromethane, trichlorotrifluoroethane,diethyl ether and dioxane or a mixture thereof. Preferably, theextraction liquid is chosen such that the spinning solvent can beseparated from the extraction liquid for recycling.

The process according to the invention further comprises drawing thesolid filaments during and/or after said removal of the solvent.Preferably, the drawing of the solid filaments is performed in at leastone drawing step with a draw ratio DR_(solid) of preferably at least 4.More preferably, DR_(solid) is at least 7, even more preferably at least10, yet even more preferably at least 15, yet even more preferably atleast 20, yet even more preferably at least 30, most preferably at least40. More preferably, the drawing of solid filaments is performed in atleast two steps, even more preferably in at least three steps.Preferably, each drawing step is carried out at a different temperaturethat is preferably chosen to achieve the desired drawing ratio withoutthe occurrence of filament breakage. If the drawing of solid filamentsis performed in more than one step, DR_(solid) is calculated bymultiplying the draw ratios achieved for each individual solid drawingstep.

More preferably, each solid drawing step is carried out by drawing thesolid filaments while passing them continuously over a length of atleast 10 meters through a drawing oven comprising driving rolls, suchthat the residence time in the oven is at most 10 minutes. Drawing inthe oven can be easily carried out by the skilled person by adjustingthe speeds of the driving rolls supporting the filaments. Preferably,the solid filaments are passed in the oven over a length of at least 50meters, more preferably at least 100 meters, most preferably at least200 meters. The residence time of the solid filaments in the oven ispreferably at most 5 minutes, more preferably at most 3.5 minutes, evenmore preferably at most 2.5 minutes, yet even more preferably at most 2minutes, yet even more preferably at most 1.5 minutes, most preferablyat most 1 minute. The temperature in said oven may also have anincreasing profile preferably between 120 and 155° C.

By residence time is herein understood the time spent by a cross-sectionof the solid filament in the oven from the moment it enters the ovenuntil it exits it. It was surprisingly found that a shorter residencetime was needed to achieve the same drawing ratio for the UHMWPEfilaments in the process of the invention than it was possible before.Therefore, the efficiency of the process of the invention was improvedin comparison with the efficiency of known processes for producingpolyethylene fibres.

In a preferred embodiment, at least one drawing step is carried out at atemperature having an increasing profile between about 120 and about155° C. Optionally, the process of the invention may also comprise astep of removing the residual spinning solvent from the UHMWPE fibres ofthe invention, preferably, said step being subsequent to the solidstretching step. In a preferred embodiment, the residual spinningsolvent left in the UHMWPE fibre of the invention is removed by placingsaid fibre in a vacuumed oven at a temperature of preferably at most148° C., more preferably of at most 145° C., most preferably of at most135° C. Preferably, the oven is kept at a temperature of at least 50°C., more preferably at least 70° C., most preferably at least 90° C.More preferably, the removal of the residual spinning solvent is carriedout while keeping the fiber taut, i.e. the fiber is prevented fromslackening.

Preferably, the UHMWPE fibre of the invention at the end of the solventremoval step comprises spinning solvent in an amount of below 800 ppm.More preferably said amount of the spinning solvent is below 600 ppm,even more preferably below 300 ppm, most preferably below 100 ppm.

The invention further relates to a gel-spun UHMWPE fibre having atensile strength of at least 4 GPa and a creep rate as measured at 70°C. under a load of 600 MPa of at most 5×10⁻⁷ sec⁻¹. More preferably, thecreep rate of the UHMWPE fibre according to the invention is at most3×10⁻⁷ sec⁻¹, even more preferably at most 1.5×10⁻⁷ sec⁻¹, yet even morepreferably at most 0.8×10⁻⁷ sec⁻¹, yet even more preferably at most0.2×10⁻⁷ sec⁻¹, most preferably at most 0.09×10⁻⁷ sec⁻¹. The tensilestrength of the UHMWPE fibre is preferably at least 4.5 GPa, morepreferably at least 5 GPa, even more preferably at least 5.5 GPa, mostpreferably at least 6 GPa.

The UHMWPE fibre is for example obtainable by the above gel-spinningprocess. Preferably the UHMWPE fibre is obtained by the above method,but other methods of manufacturing may also be feasible.

Gel-spun UHMWPE fibres with high tenacities and improved creepresistance are known for example from EP 1,699,954, EP 0,205,960 B1, EP0,269,151, JP 5-70274, U.S. Pat. Nos. 5,115,067 and 5,246,657. A summaryof the fibres' tensile strength and creep rate values as reported by theabove cited references and the conditions defined in said referencesunder which the creep rates were measured are given in Table 1. Thereferred table further includes the creep rates and tensile strengths ofthe UHMWPE fibres of the invention (Example 1) determined in accordancewith the measurement techniques and under the same conditions oftemperature and load as described in the cited references. As it can beseen from the referred table, none of the fibres of the cited referencespossesses the combination of high strength and low creep, of the UHMWPEfibres of the invention.

Preferably, the UHMWPE fibres of the invention have a modulus of atleast 100 GPa, more preferably of at least 130 GPa, even more preferablyof at least 160 GPa, yet even more preferably of at least 190 GPa, mostpreferably of at least 220 GPa. Without being bound by any theory, theinventors attributed the increase in modulus to the permissible higherDR_(overall) for the UHMWPE fibres of the invention.

The invention also relates to a yarn containing the UHMWPE fibers of theinvention.

It was observed that after processing the UHMWPE polymer used in theprocess of the invention into an UHMWPE fiber, the Δδ difference for theUHMWPE contained in the UHMWPE fiber increased. Although this effectcould not have been explained, it was surprisingly found that thiseffect contributed to achieving a fiber with improved creep rate andtensile strength properties.

In a preferred embodiment, the UHMWPE fibers of the invention contain anUHMWPE having a Δδ difference of at most 65°, more preferably at most60°, even more preferably at most 55°, yet even more preferably at most50°, yet even more preferably at most 45°, yet even more preferably atmost 42°, yet even more preferably at most 40°, yet even more preferablyat most 36°, most preferably at most 35° provided that δ₁₀₀ is at most18°. Preferably, said Δδ difference is at least 5°, more preferably atleast 15°, most preferably at least 25°. Preferably, said Δδ differencehas the above mentioned values provided that δ₁₀₀ is at most 16°, morepreferably at most 14°, even more preferably at most 12°, mostpreferably at most 10°. Preferably, δ₁₀₀ is at least 2°, more preferablyat least 4°, most preferably at least 6°.

By fibre is herein understood an elongated body, i.e. a body having alength much greater than its transverse dimensions. The fibre as usedherein includes a plurality of filaments having regular or irregularcross-sections and having continuous and/or discontinuous lengths.Within the context of the invention, a yarn is understood to be anelongated body comprising continuous and/or discontinuous fibres. Theyarn according to the invention may be a twisted or a braided yarn.

The UHMWPE fibres of the invention have properties which make them aninteresting material for use in ropes, cordages and the like, preferablyropes designed for heavy-duty operations as for example towing, marineand offshore operations. Heavy duty operations may further include, butnot restricted to, anchor handling, mooring of heavy vessels, mooring ofdrilling rigs and production platforms and the like. Most preferably,the UHMWPE fibres of the invention are used in applications where theUHMWPE fibres are experience static tension. By static tension is hereinmeant that the fibre in application always or most of the time is undertension irrespective if the tension is at constant level (for example aweight hanging freely in a rope comprising the fibre) or varying level(for example if exposed to thermal expansion or water wave motion).Examples of highly preferred used with static tension is for examplemany medical applications (for example cables and sutures), mooringropes, and tension reinforcement elements, as the reduced creep of thepresent fibres leads to highly improved system performance is these andsimilar applications.

Therefore, the invention relates to ropes containing the UHMWPE fibresof the invention. Preferably, at least 50 mass-%, more preferably atleast 75 mass-%, even more preferably at least 90 mass-% from the totalmass of the fibres used to manufacture the rope consists of the UHMWPEfibres according to the invention. Most preferably the rope consists ofthe UHMWPE fibres of the invention.

The remaining mass percentage of the fibres in the rope according to theinvention, may contain fibres or combination of fibers made of othermaterials suitable for making fibres as for example metal, glass,carbon, nylon, polyester, aramid, other types of polyolefin and thelike.

The invention further relates to composite articles containing theUHMWPE fibres according to the invention.

In a preferred embodiment, the composite article contains at least onemono-layer comprising the UHMWPE fibres of the invention. The termmono-layer refers to a layer of fibers i.e. fibers in one plane.

In a further preferred embodiment, the mono-layer is a unidirectionalmono-layer. The term unidirectional mono-layer refers to a layer ofunidirectionally oriented fibers, i.e. fibers in one plane that areessentially oriented in parallel.

In a yet further preferred embodiment, the composite article ismulti-layered composite article, containing a plurality ofunidirectional mono-layers the direction of the fibres in eachmono-layer preferably being rotated with a certain angle with respect tothe direction of the fibres in an adjacent mono-layer. Preferably, theangle is at least 30°, more preferably at least 45°, even morepreferably at least 75°, most preferably the angle is about 90°.

A mono-layer may further comprise a binder material, to hold the UHMWPEfibres together. The binder material can be applied by varioustechniques; for example as a film, as a transverse bonding strip orfibres (transverse with respect to the uni-directional fibers), or byimpregnating and/or embedding the fibers with a matrix, e.g. with asolution or dispersion of matrix material in a liquid. The amount ofbinder material is preferably less than 30 mass-% based on the mass ofthe layer, more preferably less than 20, most preferably less than 15mass-%. The mono-layer may further comprise small amounts of auxiliarycomponents, and may comprise other fibres made of materials suitable formaking fibres such as the ones enumerated hereinabove. Preferably thereinforcing fibres in the mono-layers consist of the UHMWPE fibres ofthe invention.

Multi-layered composite articles proved very useful in ballisticapplications, e.g. body armor, helmets, hard and flexible shield panels,panels for vehicle armouring and the like. Therefore, the invention alsorelates to ballistic-resistant articles as the ones enumeratedhereinabove containing the UHMWPE fibres of the invention.

The UHMWPE fibres of the invention having a low amount of residualsolvent are also suitable for use in medical devices, e.g. sutures,medical cables, implants, surgical repair products and the like.

The invention therefore further relates to a medical device, inparticular to a surgical repair product and more in particular to asuture and to a medical cable comprising the UHMWPE fibres of theinvention.

The advantage of the suture and the medical cable according to theinvention is that due to their excellent tensile properties and furtherdue to their low creep rates, these products showed a good retention oftheir mechanical properties inside the human body.

The number and the thickness of the filaments in the UHMWPE fibreaccording to the invention can vary extensively, depending on theapplication in which the fibres are to be used. For example, in case ofheavy-duty ropes for use in marine or offshore operations preferablyfibres having at least 1500 dtex, more preferably of at least 2000 dtex,most preferably of at least 2500 dtex are used. When the fibres are usedin medical devices, preferably their titer is at most 1500 dtex, morepreferably at most 1000 dtex, most preferably at most 500 dtex.

It was also observed that the UHMWPE fibres of the invention showing theabove mentioned unique combination of mechanical properties are suitablefor use in other applications like for example, fishing lines andfishing nets, ground nets, cargo nets and curtains, kite lines, dentalfloss, tennis racquet strings, canvas (e.g. tent canvas), nonwovencloths and other types of fabrics, webbings, battery separators,capacitors, pressure vessels, hoses, automotive equipment, powertransmission belts, building construction materials, cut and stabresistant and incision resistant articles, protective gloves, compositesports equipment such as skis, helmets, kayaks, canoes, bicycles andboat hulls and spars, speaker cones, high performance electricalinsulation, radomes, and the like. Therefore, the invention also relatesto the applications enumerated above containing the UHMWPE fibers of theinvention.

The invention also relates to the use of an UHMWPE as that used in theprocess of the invention in a spinning process to produce UHMWPE fibers.Such UHMWPE is characterized by a difference in the phase angleaccording to Formula 1

Δδ=δ_(0.001)−δ₁₀₀  (1)

of at most 42°, wherein δ_(0.001) is the phase angle at an angularfrequency of 0.001 rad/sec; and δ₁₀₀ is the phase angle at an angularfrequency of 100 rad/sec as measured with a frequency sweep dynamicrheological technique at 150° C. on a 10% solution of UHMWPE in paraffinoil, provided that δ₁₀₀ is at most 18°, as well as embodiments andpreferred sub ranges of the UHMWPE as described above. In oneembodiment, the spinning process is a melt spinning process, wherein theUHMWPE fibers are spun from a melt of the UHMWPE or a gel spinningprocess as described above. More preferably, the spinning process is agel-spinning process wherein the UHMWPE fibers are spun from a solutionof the UHMWPE in a solvent suitable to dissolve the UHMWPE. Mostpreferably, the gel spinning process is the process of the invention.

Hereinafter the figures are explained:

FIG. 1: Shows a cross section of the rheometer's plate system (100)provided with liquid-lock used for frequency sweep dynamic rheologicalmeasurements. The geometry of the upper (101) and lower (102) platesensures that the environment (500) is not in direct contact with thedisk sample (200) positioned between the plates. The paraffin bath (300)seals the disk sample from the environment, also ensuring a saturatedatmosphere (400) with the paraffin's vapors.

FIG. 2: Shows the variation of the phase angle δ [°] characteristic toUHMWPE grade GUR 4170 (sold by Ticona) within the angular frequency ω[rad/s] range between 0.001 rad/s and 100 rad/s.

FIG. 3: Is a schematic representation of the device used for creepmeasurements. The illustrations (1) and (2) represent an instance of theyarn length (200) at the beginning of the experiment and an instance ofthe elongated yarn after a certain time t, respectively.

FIG. 4: Shows a plot of the creep rate [1/s] on a logarithmic scale vs.the elongation in percentage [%] characteristic to the yarn of theComparative Experiment.

FIG. 5: Shows the NMR spectrum of the UHMWPE (grade GUR 4170 fromTicona) used in the process to produce the fiber of Example 1.

The invention will be further explained by the following examples andcomparative experiment.

Methods:

-   -   IV: the Intrinsic Viscosity is determined according to method        PTC-179 (Hercules Inc. Rev. Apr. 29, 1982) at 135° C. in        decalin, the dissolution time being 16 hours, with DBPC as        anti-oxidant in an amount of 2 g/l solution, by extrapolating        the viscosity as measured at different concentrations to zero        concentration;    -   Dtex: fibers' titer (dtex) was measured by weighing 100 meters        of fiber. The dtex of the fiber was calculated by dividing the        weight in milligrams to 10;    -   Tensile Properties: tensile strength (or strength) and tensile        modulus (or modulus) are defined and determined on multifilament        yarns as specified in ASTM D885M, using a nominal gauge length        of the fibre of 500 mm, a crosshead speed of 50%/min and Instron        2714 clamps, of type “Fibre Grip D5618C”. On the basis of the        measured stress-strain curve the modulus is determined as the        gradient between 0.3 and 1% strain. For calculation of the        modulus and strength, the tensile forces measured are divided by        the titre, as determined by weighing 10 metres of fibre; values        in GPa are calculated assuming a density of 0.97 g/cm³.    -   Side chains: The amounts of methyl groups corresponding to ethyl        side groups, and of methyl end groups contained by the UHMWPE        were determined by proton ¹H liquid-NMR, hereafter for        simplicity l-NMR, as follows:        -   a) 3-5 mg of UHMWPE were added to a 800 mg            1,1′,2,2′-tetracholoroethane-d2 (TCE) solution containing            0.04 mg 2,6-di-tert-butyl-paracresol (DBPC) per gram TCE.            The purity of TCE was >99.5% and of DBPC >99%.        -   b) The UHMWPE solution was placed in a standard 5 mm NMR            tube which was then heated in an oven at a temperature            between 140°-150° C. while agitating until the UHMWPE was            dissolved.        -   c) The NMR spectrum was recorded at 130° C. with a high            field 400 MHz) NMR spectrometer using an 5 mm inverse            Probehead and set up as follows: a sample spinrate of            between 10-15 Hz, the observed nucleus—¹H, the lock            nucleus—²H, a pulse angle of 90°, a relaxation delay of 30            sec, the number of scans was set to 1000, a sweep width of            20 ppm, a digital resolution for the NMR spectrum of lower            than 0.5, a total number of points in the acquired spectrum            of 64k and a line broadening of 0.3 Hz. FIG. 5 shows the NMR            spectrum of the UHMWPE of Example 1        -   d) The recorded signal intensity (arbitrary units) vs. the            chemical shift (ppm), hereafter spectrum 1, was calibrated            by setting the peak corresponding to TCE at 5.91 ppm (not            shown in FIG. 5). The TCE peak can be distinguished easily,            said peak being the highest in the ppm range between 5.5 and            6.5 in said spectrum 1.        -   e) Under identical sample preparation and experimental            conditions as described in a)-d), the spectrum, hereinafter            spectrum 2 (not shown), of an LLDPE (LLDPE 0026BP14 from            Sabic) containing only ethyl branches and end groups was            recorded.        -   f) The positions of the three peaks, i.e. a triplet,            corresponding to methyl groups of the ethyl side groups were            determined from spectrum 2, the three peaks being the            highest in the ppm range of the spectrum between 0.8 and            0.9. Said peak positions were determined at about 0.83,            about 0.85 and about 0.86, respectively.        -   g) The three peaks corresponding to methyl groups of the            ethyl side groups in spectrum 1 were identified as the peaks            at the positions determined in f) using spectrum 2. In FIG.            5, said peaks are (101), (102) and (103).        -   h) All the other peaks in the ppm range between 0.8 and 0.9            were considered as corresponding to methyl end groups. In            FIG. 5 these are (201), (202) and (203).        -   i) The deconvolution of the peaks was performed using a            standard ACD software produced by ACD/Labs;        -   j) The accurate determination of the areas (A₁            _(ethyl side groups) , hereafter A₁ and A₂ _(LCB+end groups)            , hereafter A₂) of the deconvoluted peaks corresponding to            methyl groups of the ethyl side groups and to the methyl end            groups was performed with the same software. Herein,

${A_{1} = {\sum\limits_{i = 1}^{3}\; A_{i}}},$

-   -   -    wherein A_(i) are the areas of the three peaks determined            in g) and

${A_{2} = {\sum\limits_{j = 1}^{n}\; A_{j}}},$

-   -   -    wherein n is me total number of the peaks with area A_(j)            that are different than the peaks corresponding to methyl            groups of the ethyl side groups. In FIG. 5, A_(i) are (30 i)            where i is from 1 to 3 and A_(j) are (40 j) where j is from            1 to 3.        -   k) The amounts of methyl groups of the ethyl side groups and            methyl end groups per thousand carbon atoms were computed as            follows:

${{{methyl}\mspace{14mu} {groups}\mspace{14mu} {of}\mspace{14mu} {the}\mspace{14mu} {ethyl}\mspace{14mu} {side}\mspace{14mu} {groups}} = {2 \times \frac{1000 \times \frac{A_{1}}{3}}{A_{1} + A_{2} + A_{3}}}};$${{methyl}\mspace{14mu} {end}\mspace{14mu} {groups}} = {2 \times \frac{1000 \times \frac{A_{2}}{3}}{A_{1} + A_{2} + A_{3}}}$

-   -   -    wherein A₃ is the area of the peak given by the CH₂ groups            of the main UHMWPE chain, being the highest peak in the            entire spectrum 1 and located in the ppm range of between            1.2 and 1.4 (not shown in FIG. 5).

Δδ Measurements

A 10 wt % suspension of UHMwPE powder in paraffin oil (Shell Ondina 68)and stabilizer (DBPC, 2 g/l) was preheated at a temperature of 60° C. 15grams of the preheated suspension was mixed for 5 minutes in amidi-extruder (Xplore 15 ml Micro-Compounder with standard screw set) ata temperature of 210° C., the screws rotating at a speed of 60 rpm. Themixing was performed under nitrogen.

The obtained UHMWPE solution was pressed for 10 minutes at 150° C. intoa square plate of 7.3 grams, with dimensions of 110×55 mm and athickness of 1.6 mm. The plate was subsequently allowed to cool in openair. From the plate, a disc sample was cut having a diameter of 25 mm.

Frequency sweep dynamic rheological measurements were performed in shearat 150° C. on an ARES (TA Instruments) rheometer with parallel plategeometries of 25 mm diameter. The measurements were carried out in astrain controlled mode, i.e. strain is applied on the sample. Therheometer was equipped with a plate system provided with a liquid-lockfilled with paraffin oil (FIG. 1), a high resolution actuator and a 2000grams torque (force rebalance) transducer.

Before the start of the rheological measurements the rheometer's ovenwas preheated to 100° C. with a flow of nitrogen gas (forced convectionoven). The disc sample was loaded at 100° C., after which theplate-plate distance was reduced to about 7-8 mm. The oven temperaturewas subsequently set to 150° C. and the oven was allowed to equilibrate.After about 5 minutes ensuing the equilibration of the oven, theplate-plate distance was reduced to 2.6 mm. After 1 minute the distancewas reduced again to 2 mm. Subsequently, the distance was reduced againin steps of 0.1 mm/minute until a plate-plate distance of 1.6 mm wasachieved.

The measurements started with a time sweep experiment performed for 5hours with a strain amplitude of 2% and a frequency of 0.1 rad/s. Thetime sweep experiment was intended to release the sample from theresidual stresses built during the initial sample preparation.

Immediately after ending the time sweep experiment, the frequency sweepexperiment was started. The following settings for the rheometer wereused:

-   -   The frequency interval of the frequency sweep was between 100        rad/s and 0.001 rad/s with 3 equally spaced frequencies per        decade and decreasing frequency;    -   Temperature 150° C.;    -   Initial strain amplitude (γ_(o)) 1% of the plate-plate distance;    -   Auto strain mode set ON, allowing a maximum torque of 100 g*cm,        a minimum torque of 0.5 g*cm with a strain adjustment of 90% of        current strain. Maximum allowed strain amplitude was 10% of the        plate-plate distance;    -   Auto tension mode was set on OFF.

The time and frequency sweep experiments are standard measurements beingdescribed in the ARES rheometer's operating manual.

The phase angle δ characteristic to the UHMWPE at a particular angularfrequency ω was derived according to the theoretical descriptiondetailed in “Rheology; Principles, Measurements and Applications”, 1994,VCH Publishers, Inc., ISBN 1-56081-579-5, at pages 121-123, formulas3.3.15 to 3.3.18 included as reference herein. The variation of thephase angle δ [°] characteristic to UHMWPE grade GUR 4170 (sold byTicona) within the frequency ω [rad/s] range between 0.001 rad/s and 100rad/s is shown in FIG. 2.

Creep Measurements

Creep tests were performed with a device as schematically represented inFIG. 3, on untwined yarn samples, i.e. yarn with substantially parallelfilaments, of about 1500 mm length, having a titer of about 504 dtex andconsisting of 64 filaments.

The yarn samples were slip-free clamped between two clamps (101) and(102) by winding each of the yarn's ends several times around the axesof the clamps and then knotting the free ends of the yarn to the yarn'sbody. The final length of the yarn between the clamps (200) was about180 mm.

The clamped yarn sample was placed in a temperature-controlled chamber(500) at a temperature of 70° C. by attaching one of the clamps to thesealing of the chamber (501) and the other clamp to a counterweight(300) of 3162 g resulting in a load of 600 MPa on the yarn. The positionof the clamp (101) and that of clamp (102) can be read on the scale(600) marked off in centimeters and with subdivisions in mm with thehelp of the indicators (1011) and (1021).

Special care was taken when placing the yarn inside said chamber toensure that the segment of the yarn between the clamps does not touchany components of the device, so that the experiment can run fullyfriction free.

An elevator (400) underneath the counterweight was used to raise thecounterweight to an initial position whereat no slackening of the yarnoccurs and no initial load is applied to the yarn. The initial positionof the counterweight is the position wherein the length of the yarn(200) equals the distance between (101) and (102) as measured on (600).

The yarn was subsequently preloaded with the full load of 600 MPa during10 seconds by lowering the elevator, after which the load was removed byraising again the elevator to the initial position. The yarn wassubsequently allowed to relax for a period of 10 times the preloadingtime, i.e. 100 seconds.

After the preloading sequence, the full load was applied again. Theelongation of the yarn in time was followed on the scale (600) byreading the position of the indicator (1021). The time needed for saidindicator to advance 1 mm was recorded for each elongation of 1 mm untilthe yarn broke.

The elongation of the yarn ε₁ [in mm] at a certain time t is hereinunderstood the difference between the length of the yarn between theclamps at that time t, i.e. L(t), and the initial length (200) of theyarn L₀ between the clamps. Therefore:

ε_(i)(t)[in mm]=L(t)−L ₀

The elongation of the yarn [in percentages] is:

${{ɛ_{i}(t)}\lbrack {{in}\mspace{14mu} \%} \rbrack} = {\frac{{L(t)} - L_{0}}{L_{0}} \times 100}$

The creep rate [in 1/s] is defined as the change in yarn's length pertime step and was determined according to Formula (2) as:

$\begin{matrix}{{\overset{.}{ɛ}}_{i} = {\frac{ɛ_{i} - ɛ_{i - 1}}{t_{i} - t_{i - 1}} \times \frac{1}{100}}} & (2)\end{matrix}$

wherein ε_(i) and ε_(i-1) are the elongations [in %] at moment i and atthe previous moment i−1; and t, and t_(i-1) are the time (in seconds)needed for the yarn to reach the elongations ε_(i) and ε_(i-1),respectively. The creep rate [1/s] was then plotted on a logarithmicscale vs. the elongation in percentage [%] as for example shown in FIG.4 for the yarn of the Comparative Experiment. The minimum of the curvein FIG. 4 was then used as the creep rate value characteristic to theinvestigated yarn.

COMPARATIVE EXAMPLE 1

A 5 mass-% solution of a UHMWPE in decalin was made, said UHMWPE havingan IV of 21 dl/g as measured on solutions in decalin at 135° C. TheUHMWPE had a Δδ of 46° for δ₁₀₀ of 14°.

The UHMWPE solution was extruded with a 25 mm twin screw extruderequipped with a gear-pump at a temperature setting of 180° C. through aspinneret having a number n of 390 spinholes into an air atmospherecontaining also decalin and water vapors with a rate of about 1.5 g/minper hole.

The spinholes had a circular cross-section and consisted of a gradualdecrease in the initial diameter from 3.5 mm to 1 mm with a cone angleof 60° followed by a section of constant diameter with L/D of 10, thisspecific geometry of the spinholes introducing a draw ratio in thespinneret DR_(sp) of 12.25.

From the spinneret the fluid fibres entered an air gap of 25 mm and intoa water bath, where the fluid fibres were taken-up at such rate that atotal draw ratio of the fluid UHMWPE filaments DR_(fluid) of 277 wasachieved.

The fluid fibres were cooled in the water bath to form gel fibres, thewater bath being kept at about 40° C. and wherein a water flow was beingprovided with a flow rate of about 50 liters/hour perpendicular to thefibres entering the bath. From the water bath, the gel fibres weretaken-up into an oven at a temperature of 90° C. wherein solventevaporation occurred to form solid fibres.

The solid fibres were drawn in the oven by applying a draw ratio ofabout 26.8, during which process most of the decalin evaporated.

The total stretch ratio DR_(overall) (=DR_(fluid)×DR_(gel)×DR_(solid))amounted 277×1×26.8=7424.

EXAMPLE 1

The Comparative Experiment was repeated with an UHMWPE (GUR 4170 fromTicona) having an IV of about 34 dl/g and a Δδ of 38° for δ₁₀₀ of 14°.

EXAMPLE 2

Example 1 was repeated with a fluid draw ratio of 345 and a draw ratioapplied to the solid fibres of 26. The same geometry of the spinneret asin the Comparative Example was used.

EXAMPLE 3

Example 1 was repeated with a fluid draw ratio of 350 and a draw ratioapplied to the solid fibres of 33. The same geometry of the spinneret asin the Comparative Example was used.

EXAMPLE 4

Example 1 was repeated with a fluid draw ratio of 544 and a draw ratioapplied to the solid fibres of 36. The spinneret contained spinholeshaving a gradual decrease in the initial diameter from 3.5 mm to 0.8 mmwith a cone angle of 60° followed by a section of constant diameter withL/D of 10, this specific geometry of the spinholes introducing a drawratio in the spinneret DR_(sp) of 19.1.

EXAMPLE 5

Example 4 was repeated with a fluid draw ratio of 615 and a draw ratioapplied to the solid fibres of 32.

EXAMPLE 6

Example 4 was repeated with a fluid draw ratio of 753 and a draw ratioapplied to the solid fibres of 32.

The fibres' properties of the Comparative Example and of the Examples,i.e. creep rate, tensile strength, and modulus are summarized in Table2. From said table it can be seen that by increasing the DR_(overall)fibers with better mechanical properties in terms of strength and creepcan be produced. Said table further shows that by using the sameprocessing parameters but the UHMWPE according to the invention, fiberswith improved mechanical properties are obtained as compared with fibersmade from known polyethylenes.

TABLE 1 UHMWPE fibres of Fibres of the cited documents Example 1(reported units) (transformed units) Measurement conditions TensileTensile Temperature strength strength Document ° C. Load Creep value GPaCreep value GPa EP 1,699,954 70° C. 600 MPa 0.91 × 10⁻⁶ 1/s  4.1 4.1 ×10⁻⁷ 1/s  4.3 EP 0,205,960 71.1° C.   2758.3 kg/cm² = 0.08%/hour <3.40.0018%/hour 270.6 MPa EP 0,269,151 50° C. 600 MPa  5 × 10⁻⁸ 1/s 2.7 2 ×10⁻⁸ 1/s JP 5-70274 50° C. 770 MPa 1.1 × 10⁻⁸ 1/s 3.7 7 × 10⁻⁸ 1/s U.S.Pat. No. 70° C. 30% of 2.18 9.5 × 10⁻⁶ 1/s 2.18 4 × 10⁻⁷ 1/s 5,115,067GPa = 654 MPa U.S. Pat. No. 70° C. 30% of 3.1 2.89 × 10⁻⁵ 1/s  3.1 2 ×10⁻⁶ 1/s 5,246,657 GPa = 930 MPa

TABLE 2 TS Modulus Creep-rate n DR_(sp) DR_(ag) DR_(fluid) DR_(solid)DR_(overall) (GPa) (GPa) ×10⁻⁷ (sec⁻¹) Comp. Ex. 390 12.25 22.6 277 26.87424 4.1 160 9.1 Ex 1 390 12.25 22.6 277 26.8 7424 4.35 172 4.1 Ex 2 39012.25 28.2 345 26 8970 4.5 175 2.1 Ex 3 390 12.25 28.5 350 33 11550 4.85189 0.8 Ex 4 390 19.1 28.5 544 36 19584 5.1 205 0.56 Ex 5 390 19.1 29.6615 32 19680 5.3 208 0.13 Ex 6 390 19.1 39.4 753 32 24096 5.35 208 0.091

1. A process for producing gel-spun UHMWPE fibres having high tenacitiesand improved creep resistance, the process comprising the steps of: (a)preparing a solution of an UHMWPE in a solvent, said UHMWPE having anintrinsic viscosity in decalin at 135° C. of at least 5 dl/g; (b)spinning the solution of step a) through a spinneret containing multiplespinholes into an air gap to form fluid filaments; (c) cooling the fluidfilaments to form solvent-containing gel filaments; and (d) removing atleast partly the remaining solvent from the gel filaments to form solidfilaments before or during drawing the solid filaments; wherein theUHMWPE possesses a difference in the phase angle of at most 42°according to Formula 1Δδ=δ_(0.001)−δ₁₀₀  (1) where, δ_(0.001) is the phase angle at an angularfrequency of 0.001 rad/sec; and δ₁₀₀ is the phase angle at an angularfrequency of 100 rad/sec as measured with a frequency sweep dynamicrheological technique at 150° C. on a 10% solution of UHMWPE in paraffinoil, provided that δ₁₀₀ is at most 18°.
 2. The process of claim 1,wherein the UHMWPE possesses a difference Δδ of at most 40° providedthat δ₁₀₀ is at most 18°.
 3. The process of claim 1, whereinDR_(overall)=DR_(fluid)×DR_(gel)×DR_(solid) is at least
 5000. 4. Theprocess of claim 1, wherein DR_(fluid)=DR_(sp)×DR_(ag) is at least 100.5. The process of claim 1, wherein the solid filaments are drawn in atleast one step with a solid drawn ratio DR_(solid) of at least
 4. 6. Amelt-spinnable or gel-spinnable ultrahigh molecular weight polyethylene(UHMWPE) which possesses a difference in the phase angle of at most 42°according to Formula 1:Δδ=δ_(0.001)−δ₁₀₀  (1) wherein δ_(0.001) is the phase angle at anangular frequency of 0.001 rad/sec; and δ₁₀₀ is the phase angle at anangular frequency of 100 rad/sec, as measured with a frequency sweepdynamic rheological technique at 150° C. on a 10% solution of UHMWPE inparaffin oil, provided that δ₁₀₀ is at most 18°.
 7. A fiber formed ofthe UHMWPE according to claim
 6. 8. The fiber according to claim 7,wherein the fiber is under static tension.